So calling a special function with a specific policy is just a matter of
defining the policy type to use and passing it as the final parameter.
For example, suppose we want tgamma
to behave in a C-compatible fashion and set ::errno when an error occurs, and never
throw an exception:

Alternatively, for ad hoc use, we can use the make_policy
helper function to create a policy for us: this usage is more verbose,
so is probably only preferred when a policy is going to be used once only: